Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. Alternatively, in a color flow imaging mode, the movement of fluid (e.g., blood) or tissue can be imaged. Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The frequency shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. In power Doppler imaging, the power contained in the returned Doppler signal is displayed.
Conventional ultrasound transducers transmit a broadband signal centered at a fundamental frequency f.sub.0, which is applied separately to each transducer element making up the transmit aperture by a respective pulser. The pulsers are activated with time delays that produce the desired focusing of the transmit beam at a particular transmit focal position. As the transmit beam propagates through tissue, echoes are created when the ultrasound wave is scattered or reflected off of the boundaries between regions of different density. The transducer array is used to transduce these ultrasound echoes into electrical signals, which are processed to produce an image of the tissue. These ultrasound images are formed from a combination of fundamental and (sub) harmonic signal components, the latter of which are generated in a nonlinear medium such as tissue or a blood stream containing contrast agents.
In certain instances ultrasound images may be improved by suppressing the fundamental and emphasizing the (sub)harmonic (nonlinear) signal components. If the transmitted center frequency is at f.sub.0, then tissue/contrast nonlinearities will generate harmonics at kf.sub.0, where k is an integer greater than or equal to 2. Also, subharmonics at frequencies f.sub.0 /k may be generated by contrast bubble destruction. Imaging of (sub)harmonic signals has been performed by transmitting a narrowband signal at frequency f.sub.0 and receiving at a band centered at frequency 2f.sub.0 (second harmonic) or at frequency f.sub.0 /2 (subharmonic) followed by receive signal processing.
Conventional medical ultrasound scanners use one-dimensional (1D) arrays containing N transducer elements that may be multiplexed and/or electronically steered and focused via phased array techniques. Usually a half wavelength (LAMBDA/2) spacing is required between elements in order to avoid producing undesirable grating lobes in the array response. This is why a large number of array elements (e.g., N=128) must be employed in realistic imaging systems. Even then, a linear array is limited to focusing and steering only in the array direction.
In recent years, N.times.M matrix arrays have been introduced on state-of-the-art ultrasound scanners. The multiple (M&gt;1) rows of elements enable electronic focusing in the elevation dimension, thereby producing much improved contrast resolution over an extended axial range. However, the number of rows in these matrix arrays is still quite limited (M&lt;10), and they do not support beam steering in the elevation dimension. For this reason, they are referred to as "1.5D" arrays.
The development of true 2D transducer arrays which allow focusing and beam steering in both lateral dimensions continues to receive much attention because such transducer arrays can potentially enable real-time 3D volumetric imaging, and other advanced imaging techniques, such as phase aberration correction. One of the primary challenges in realizing a 2D array is that the number of beamformer channels (with A/D converters and focusing delays) required becomes prohibitively large. For example, a 64.times.64 element array requires over 4000 beamformer channels. For this reason, much research effort has been devoted towards development of methods for reducing the number of elements in a 2D array, while maintaining the beam properties similar to those obtained with the full 2D array. As investigated by Turnbull and Foster ("Beam steering with pulsed two-dimensional transducer arrays," IEEE Trans. Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 18, pp. 320-333, 1991), one method is to use a circular rather than a square aperture, which can reduce the element count by about 25%. A more aggressive element reduction scheme, adapted from radar antenna ray theory, is to remove a random selection of elements from the periodic dense array, such that the resultant "sparse array" contains only a small fraction (e.g., 15%) of the original elements. As used herein, the term "sparse array" refers to an array of active transducer elements arranged on a grid comprising a multiplicity of periodically spaced grid points, wherein only a fraction of the grid points are occupied by active transducer elements. The grid points may be arranged in a square, rectangular, hexagonal, triangular or any other suitable geometric pattern. In the case where a transducer element is placed at every grid point, i.e., a so-called "fully populated array", less than all of the elements are activated during either transmit or receive. Alternatively, the sparse array can be formed by forming transducer elements at only the selected grid points making up the sparse array, leaving the remaining grid points unoccupied. The former case is preferred due to the ability to adapt the number and placement of transducer elements by activating a selected subset of elements for a given transmit or receive aperture.
One drawback of sparse arrays is that the large spacing between active elements will invariably give rise to high sidelobes and/or grating lobes which can cause major degradations in image quality. In general, the average sidelobe level will increase with the number of elements removed. To reduce these undesirable effects, a number of sparse array strategies have been proposed which may involve either random or intelligent selection of active elements. The optimal configuration depends on the goodness criteria, such as minimizing the peak sidelobe, minimizing the first sidelobes and minimizing the difference between the response of the actual array and a dense array response. Unfortunately, none of these strategies can keep all sidelobes to within acceptable levels under all scan situations.
There is a need for a method and an apparatus for enhancing the sidelobe performance of a sparse ultrasonic transducer array.